Generally, an image reader includes a case made of a synthetic resin and certain parts mounted in the case (See JP No. 2004-193773, for example). An example of conventional image reader is shown in FIGS. 7 and 8.
Main structural parts of the conventional image reader will be described with reference to FIG. 7. The image reader B includes a case 101. A transparent cover 102 and a substrate 103 in the form of an elongated plate are mounted to an upper portion and a lower portion of the case 101, respectively. A light source 104 is mounted at an end of the substrate 103, and a plurality of sensor IC chips 108 are mounted on the substrate 103 to be aligned longitudinally of the substrate 103. A light guide 105, a reflector 106 and a lens array 107, all of which are elongate longitudinally of the case 101, are arranged in the case 101.
The operation of the image reader B will be described below. The light emitted from the light source 104 travels through the light guide 105 while being scattered little by little to the outside of the light guide 105. Thereafter, the light passes through the transparent cover 102 to irradiate the document P. The light reflected at the document P passes through the lens array 107 and converges on a plurality of light receiving elements (not shown) arranged in a sensor IC chip 108. Each of the light receiving elements outputs an electric signal corresponding to the amount of received light. By processing electric signals outputted by the light receiving elements, i.e., image signals, an image corresponding to the document P can be obtained.
With reference to FIG. 8, the structure of the image reader B will be described in more detail. The case 101 includes, at a lower portion thereof, a peripheral wall 101a and a recess 101b provided inward of the peripheral wall 101a. The substrate 10 is fitted into the recess 101b to be mounted to the case 101. The peripheral wall 101a also serves to prevent external light or a foreign matter from entering the case 101 through a gap between the case 101 and the substrate 103.
A connector 109 is provided at one of longitudinally opposite ends of the substrate 103. The connector 109 is used for connecting the substrate 103 to an external device. The connector 109 partially projects out from a longitudinal edge of the substrate 103. The peripheral wall 101a of the case 101 includes a cutout 101d. The cutout 101d is provided for preventing the outwardly-projecting connector 109 from coming into contact with the peripheral wall 101a in mounting the substrate 103 to the case 101.
Generally, the dimension of the cutout 101d is set larger than the minimum dimension required for preventing the connector 109 from coming into contact with the peripheral wall 101a. One of the purposes of this is to reliably prevent the contact between the connector 109 and the peripheral wall 101a, and the other purpose is to make it possible to use various kinds of connectors and make the structural parts of the image reader B be widely usable. Due to this structure, when the substrate 103 is mounted to the case 101, a relatively large gap c1 is defined between the cutout 101d and the connector 109. This gap c1 allows light and foreign matters from outside to enter the case 101. To prevent this, the case 101 further includes a partition wall 101e integrally formed on the case 101, as shown in FIG. 8. The partition wall 101e separates the light receiving elements from the cutout 101d so that light and foreign matters are prevented from coming close to the light receiving elements in the case 101.
Described above is the structure of a conventional image reader. To be described below are problems which occur in actually using an image reader.
Image signals are liable to be influenced by electric noise. The inclusion of noise in image signals deteriorates the image quality. Therefore, in the actual use of the device, it is necessary to take measures to reduce the influence of electric noise.
As one of typical countermeasures against noise, a conductive housing can be employed as a housing for surrounding the electric circuit and the system, and the housing is connected to the ground wiring. With this method, the housing is prevented from being charged, which may lead to the enhancement of resistance to noise.
FIG. 9 shows the image reader B to which the above-described countermeasure against noise is applied. The case 101 is conductive and made of polycarbonate containing carbon fiber, for example. Electrodes 110A and 110B are provided on an upper surface of the substrate 103 and a lower end surface of the partition wall 101e of the case 101, respectively. For instance, these electrodes are formed by applying silver paste having excellent conductivity to predetermined portions. The electrode 110A is connected to a ground wiring (not shown) provided on the substrate 103. A solder bump 111 is provided on the electrode 110A. When the substrate 103 is mounted to the case 101, the solder bump 111 comes into contact with the electrode 110B.
Generally, much resin components are deposited on a surface of the case 101. Even when such a surface of the case is brought into contact with a solder bump, it is difficult to obtain sufficient conduction. Unlike this, in the example shown in FIG. 9, the highly conductive electrode 110B is provided on a case surface, and the size of the electrode 110B is considerably larger than that necessary for coming into contact with the solder bump 111. With this structure, the case 101 is electrically connected to the substrate 103 via the electrode 110B, the solder bump 111 and the electrode 110A and hence connected to ground. Therefore, the case 101 is not charged excessively, so that inclusion of noise in the image signals can be prevented.
However, the above-described countermeasure against noise causes another problem, i.e., causes silver particles to appear as dust. Specifically, for example, the silver particles contained in the electrode 110B may appear as dust due to the rubbing between the solder bump 111 and the electrode 110B. Further, the electrodes 110A and 110B may loose adhesion due to the volatilization of a solvent, whereby silver particles naturally appear from the surfaces of the electrode 110A and 110B as dust. When the dust appeared in this way scatters and adheres to the light receiving elements, the light receiving elements cannot properly detect light, which leads to the degradation of the image reading quality.